As a method for producing a silicon single crystal used for a semiconductor material, the Czochralski method (CZ method) has been widely employed. The Czochralski method is a method in which a seed crystal is dipped into and pulled upward from silicon that is melted in a quartz crucible to make a single crystal grow from the lower end of the seed crystal.
When a silicon single crystal is grown by this method, oxygen contained in the quartz crucible is dissolved in the molten silicon, and part of the oxygen is incorporated into the single crystal. Such oxygen causes a precipitate, a dislocation loop, and a stacking fault, in a heat treatment process to produce a device using a wafer cut from the silicon single crystal. Such defects have various influences on the quality of the single crystal, and appropriately controlling the concentration of oxygen to be incorporated into a single crystal is considered to be indispensable for producing a silicon single crystal with high quality.
As mentioned above, since the oxygen incorporated into the silicon single crystal during the growth of the single crystal has various influences on the quality of the single crystal, it is necessary to properly control the oxygen concentration in the single crystal. A problem about the oxygen concentration in the silicon single crystal is in particular variations in oxygen concentration in a single crystal in a growth axis direction. The oxygen concentration is influenced by various factors, and the influences of these factors change as the pulling of the crystal proceeds. For this reason, the homogeneity of the oxygen concentration in a single crystal in a growth axis direction cannot be secured, resulting in a decreased yield in producing a silicon single crystal that has a narrow required range of oxygen concentration specifications between an upper limit value and a lower limit value.
To secure the homogeneity of oxygen concentration in this silicon single crystal in a growth axis direction, for example, Patent Literature 1 discloses a pulling method for a single crystal in which the position of the bottom surface of a quartz crucible (a melt surface level) is moved up or down in accordance with the heat distribution property of a heater and a pulling length of the single crystal, and at the same time the flow velocity of inert gas introduced into an apparatus is controlled. Patent Literature 1 describes that, according to the single crystal pulling method, it is possible to obtain a silicon single crystal that has a desired oxygen concentration and a uniform axis direction oxygen distribution.